You have marked an answer as correct and it might solve what you want to do but the message "No root file system is defined" means (I think) that although you selected the free space you did not mark it to be mounted as / . So, the installer did not know where to put the root file system. You needed to Edit the partition and select an mount point and a partition format.
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grahammechanicalNov 7 '11 at 0:26

4 Answers
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At present i am Triple booting windows7,win8 developer preview and ubuntu 11.10 64 bit.If win8 is so important to you,i should tell you no way of adding a boot entry of Ubuntu in windows8 boot menu and ubuntu wont recognise windows 8(i have run down sudo update-grub and it didnt recognize win8).

So install windows7 in as small partition as possible.Use easeus partition manager or gparted live cd or any Ubuntu live cd(Try ubuntu and use gparted).

you can download gparted live here.there are also instructions to make a live cd/usb over there.

Resize your win8 partition and make a partition for win7 and some free space for Ubuntu,and install win7 in the partition.It is good to have a patition for data(so that it can be accesed in all 3 OS es,and would be convenient if Ubuntu,win7 and data are all in extended partition).this is how my partition table looks like

windows8 recognizes win7 and a menu entry of win7 will be present in win8 boot menu.Now boot into Ubuntu live cd installer and use advanced option and select free space and select add option and make a partition for /,/home,swap and install the bootloader in the respective / partition and remember the partition(like sda7).Once installed,do not restart,just close the window and open a terminal and run the following command

sudo dd if=/dev/sda7 of=/media/SHARE/ubuntu.bin bs=512 count=1

"if" is input folder path "of" is output folder path and copy the created ubuntu.bin into C drive of win7.Now boot into win7 and follow the this article.
after this just select the boot options of win8 and make win7 as default OS to boot,this would give you win7 bootmenu with win8 and ubuntu

I think I know what your problem is here. I'm sure you've received all of the warnings about partitioning and why you should back up your data but it sounds like there is nothing important on your hard drive considering it's just holding the Windows 8 dev preview. What you'll be doing here is pretty safe though:

Ubuntu is complaining that you haven't set up a root partition so we need to set up the partitions that Ubuntu will use.

Click on the "free space" entry and then click on the "Add..." button to create the swap partition that Ubuntu will need to be able to sleep/hibernate. From here use the following values:

Partition size: This should be roughly the same as the amount of RAM that you have in your system, for example I have 4GB of RAM so I have a 4GB swap partition.

Location of new partition: Beginning
Use as: swap

Now click OK and we'll move on to creating the partition that Ubuntu will actually reside on.

Simply click on "free space" again and then "Add..." and use these values:

New partition size: Leave this as default to use up all of the un-partitioned space on your drive.

Now hit "OK" again and carefully review your changes to make sure you haven't chosen to format any of your Windows partitions (commonly NTFS). When you're happy with your changes feel free to chose "Install Now". Good luck and if you're unsure feel free to ask me rather than take a risk :P

I find that Ubuntu 11.10 (and Lubuntu 11.10 for that matter, as I have tried) does not recognise Windows XP, even in live session of Ubuntu, and certainly not in the installer. I have also tried installing Ubuntu 11.10 using manual direction in the installer. This resulted in Ubuntu installing as directed however XP was ignored in the boot grub list and even then reinstalling grub from an 11.10 live CD and update-grub did not fix this. Ubuntu 11.04 is ok, it recognises XP. I wonder of Windows 8 suffers similarly with 11.10?

I was wondering if there was a way to install Win8 and Ubuntu in dual boot without going through installing Win7. I spent the whole morning trying but with no luck: I tried creating a partition from Win8, using fdisk -l (nothing happens), using sudo update-grub, etc.
Any idea?